I don't know what is going wrong in this case, but meanwhile if when you have the game showing in the normal way, you click on 'Game History' you can use this board to try out possible continuations. One warning though, it will let you make illegal moves, so check carefully before actually going into a line you have explored.

When I see ".../core/..." in the URL-adress, I think this adress is only accessable to the user himself, noone else.
To share this adress to another member, it will violate the private nature of the game, such ass info about messages.

Remedy: Find the game under "public games" and copy the URL from there. Then you don't see the ".../core/..." and it is therefore public to anyone.

I don't know if this will help you with the problem. I'm happy if it does, otherwise I have to think more...